Proposal for Relevant Comparisons of Natural and Related Reference Materials Lisa Karam, NIST 29 March 2005

Transcription

1 Proposal for Relevant Comparisons of Natural and Related Reference Materials Lisa Karam, NIST 29 March 2005 In the field of radionuclide metrology (radioactivity measurements), a particular issue has arisen with regards to CMCs of reference materials (soils, organic matrices, natural waters, etc.), most of which have not been subject to either key or supplementary comparisons. While the measurement of their contributing radionuclides (such as 137 Cs) have been compared, and such comparisons are used to support the CMC of 137 Cs in soil, the comparison of the reference materials themselves offers very specific and often recalcitrant difficulties. In addition to the preponderance of a vast variety of reference materials, many of which are considered by only one laboratory, how such material is to be handled (sampling) and prepared for analysis (i.e., procedures used to extract the nuclides of interest from the matrix quantitatively) present potential problems for any kind of comparison. The NIST has, over many years, performed comparisons (not to be considered either key or supplementary as they have not followed the guidelines for such) of reference materials as a crucial part of the development of a variety of matrix-based Standard Reference Materials (SRMs). In addition, NIST prepares, distributes, and analyzes reference materials as part of the performance evaluation program referred to as the NRIP, the NIST Radiochemistry Intercomparison Program. This is a comparison program among several (12 in 2004) laboratories in the United States with expertise in radiochemistry and the measurement of radionuclides in natural matrix materials, including many of the National Labs of the US Department of Energy (laboratories with a history of research and development with radiation and radioactive materials). Details of the NRIP are in the public domain and for example in "NIST Radiochemistry Intercomparison Program: a summary of four-year performance evaluation study", Z. Wu, K. G. Inn, Z. C. Lin, C. A. McMahon, L. R. Karam, Applied Radiation Isotopes, 56, (2002). At the present time, three reference material-based SRMs are under development at NIST. Rocky Flats II (soil), seaweed (vegetation), and shellfish meat (biological material) have been recognized by the respective user communities as of importance for environmental monitoring and oceanic studies. In addition to US laboratories participating in the analyses of these reference materials, a large variety of international labs are contributing measurement results for the characterization of these SRMs (MRA signatories are indicated in bold): SRM 4353A (Rocky Flats -II) - International Atomic Energy Agency (Monaco) - National Research Center for Environment and Health, Institute of Radiation Protection (Germany) - British Nuclear Fuels (United Kingdom) - National Institute of Standards and Technology (NIST) (USA) Seaweed SRM - Australian Nuclear Science and Technology Organization (Australia) - Australian National University (Australia) - The Centre for Environment, Fisheries and Aquaculture Science (United Kingdom) 1

2 - Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (Spain) - Institute for Energy Technology (Norway) - Institute of Nuclear Energy Research (Taiwan) - Interfaculty Reactor Institute (The Netherlands) - Institut de Radioprotection et de Sûreté Nucléaire (France) - IsoTrace Laboratory (Canada) - Institute for Transuranium Elements (Germany) - Korea Ocean Research and Development Institute (Korea) - Lund University Department of Radiation Physics (Sweden) - Agricultural University (Norway) - National Physical Laboratory (United Kingdom) - National Radiological Protection Board (United Kingdom) - Physikalisch-Technische Bundesanstalt (Germany) - Risø National Laboratory (Denmark) (?) - Southampton Oceanography Centre (United Kingdom) - Radiation and Nuclear Safety (Finland) - Scientific Production Association "Typhoon" (Russia) - Universitat Autónoma de Barcelona (Spain) - National Institute of Standards and Technology (NIST) (USA) Shellfish SRM - Institute for Reference Material & Measurements (Belgium) - Institute of Oceanography (Poland) - Taiwan Radiation Monitoring Center (Taiwan) - Bedford Institute of Oceanography (Canada) - Laboratoire de Mesure de la Radioactivité de l'environnement (France) - National Physical Laboratory (United Kingdom) - Institute of Nuclear Energy Research (Taiwan) - Scientific Production Association (Russia) - University College Dublin (Ireland) - National Institute of Metrology (PR China) - National Institute of Radiological Sciences (Japan) - Japan Chemical Analysis Center (Japan) - National Institute of Standards and Technology (NIST) (USA) Experiences from these types of exercises suggest how to best approach the design and implementation of comparisons of reference materials to support the CMCs. Specific points which must be considered include not only what types of matrices should be used and which radionuclides to include, but, perhaps most importantly, at what levels (ambient, spiked, high, low) should the activity be so as to verify the capability. Proposal: Rather than carrying out comparisons for each individual reference material, some sort of representation should be developed for specific materials. For example, an comparison of a high calcium content soil should support claims for other high calcium content soils or sediments. 2

3 While reference materials indicated in the Appendix C of the CIPM MRA show a wide variety of radionuclides, some attempt should be made to optimize the combination from a radiological and chemical point of view. A relatively small number, on the order of 10 to 12, carefully selected nuclides in a given matrix should be adequate to evaluate the capability for a wider range of potential analytes. Particular attention must be paid to the suite of nuclides to be considered in a reference material since, while a given nuclide might be rated as comparatively easy to quantity ( green in the generic groupings), the chemistry for its extraction might result in a sample much more difficult to analyze. Examples of matrices are given in Table 1. Table 1 Matrices Special considerations Previous and similar comparisons Soils/sediments high/low organic content high/low calcium content Rocky Flats I Ocean Sediment (high Ca) Lake Sediment (high organics) Vegetation Water content Mate tea Seaweed Biological materials (food, tissues, etc.) Water content Sample storage (refrigeration?) Milk Powder Bone Shell Fish Aqueous high/low salt content Synthetic Urine (high salt) Building materials Inhomogeneity in composition Spiked Water (low salt) None yet Experience has shown that the general process of allowing the measuring laboratory to choose its preferred sample preparation and analytical methods, with the stipulation that the details of said preparation and analyses are documented, is the preferred approach to assure that the comparison laboratory uses its most experienced and best-documented procedure. In general, the same type of information requested for any radionuclide comparison (reference time, half lives used, radionuclides measured, etc.) apply to comparisons of reference materials. It may also be appropriate to specify certain other points such as defining the sample size and yield monitor to be used, see Table 2. Table 2 Information to be Included on Data Sheet Counting time Chemical yield Interferences Considerations Time needs to be long enough for good statistics (may take longer than conventional comparisons) Must be certain to trace the entire extraction process Especially important when a suite of radionuclides is involved (similar to any mixed-nuclide source) Responsibility of Lead Laboratory (before distribution to participants) Counting time appropriate for the specific material should be determined Leachability and total sample digestion (or non-destructive analysis) to confirm suitability of material for use 3

4 Software Measurement method(s) Efficiencies Sample size Radiochemical methodologies Yield monitor (tracer) Low level measurements (i.e., lower limit) For measurement and statistical analysis Standard approaches; perhaps confirmatory measurements are particularly useful? Usual issues, complicated by interferences Sampling, verification of subsample homogeneity, checking for hot spots, etc. Source handling (e.g., combustion to remove organics, etc.), chemical preparation, etc. Appropriateness (chemical and radiological) and traceability of monitoring tracer The minimum value (in the CMC) would be the lowest level obtained in the comparison quoted State to participants the software used to evaluate the material before distribution Validate an homogenous sample, or assess the degree of heterogeneity Recommend procedures for storage of material, sub-sampling, and drying of material; determine particle size (soils/sediments and other solids), etc. Recommend chemically-appropriate tracer to be used Several items should be considered before a comparison of reference materials is designed. Firstly, it is very unlikely that measurements of these types of material could reasonably provide an activity value only an activity per mass or per volume. This has as much to do with the nature of large samples with very low activity levels as with the vagaries of extraction processes. Therefore, the first recommendation would be to remove any CMC which states an activity measurement for a reference material, and to not accept any in the future. A second issue to be addressed is the handling of those CMCs which address a unique object or material (such as metal spoons, metal discs or semiconductors) which probably should not be considered as actual reference materials. While the measurements of such items presents interesting research possibilities, they are too specialized to be useful to the community at large. Since the sample preparation of such materials would be extremely complex, no comparisons of other reference materials (soils/sediments, flora, water, etc.) would be applicable to support claims for these items. Therefore, CMCs of specialized materials should not be acceptable in Appendix C. After considering these two issues, and eliminating efforts for them, the list of CMCs for matrix reference materials can be narrowed down to those specified in Table 3. 4

5 Table 3 Type of material NMIs claiming CMCs Radionuclides Foods CNEA, IRMM Co-57, Cs-134, Cs-137, K- 40, Mn-54, Na-22, Zn-65 Water (aqueous) NIST, LNMRI Am-241, Ba-133, Co-57, Co- 58, Co-60, Cs-134, Cs-137, Mn-54, Na-22, Pu-238, Pu- 240, Ra-228, Sr-90, Th-230, Biological materials Soils/sediments NIST, BEV, IRMM NIST, BEV, IRMM U-234, U-238, Zn-65 Ac-228, Am-241, Co-60, Cs- 137, Pb-210, Pu-238, Pu , Ra-226, Sr-90, Th- 228,Th-230, Th-232, Tl-208, U-234, U-238 Ac-228, Al-26, Am-241, Ba- 133, Cd-109, Co-57, Co-58, Co-60, Cd-134, Cs-137, Eu- 152, Eu-154, Mn-54, Pu-238, Pu , Sr-85, Sr-90, Th-228, Th-230, Th-232, U- 234, U-238, Y-88, Zn-65 Flora (vegetation) CNEA, NIST Co-57, Cs-134, Cs-137, Mn- 54, Na-22, Zn-65 Building materials Other BEV, IRMM Am-241, Ba-133, Cd-109, Ce-139, Co-57, Co-60, Cs- 134, Cs-137, Mn-54, Pb-210, Ra-226, Sb-125, Sr-85, Th- 234, U-235, Y-88, Zn-65 Matrices Grains, soy meal, milk powder Spiked water Synthetic spiked urine, human liver, human lung, ashed bone, ocean shellfish High level soil (Rocky Flats), low level soil (Peruvian), river sediment, ocean sediment, spiked soil, fresh water lake sediment, bentonite (clay), quartz (sand), meteorite Mate tea (seaweed, in process) None at the present time ML-1, 2, 4; filter paper Proposed Exercise: Reference Material Choice As there seem to be more soil/sediment reference materials than anything else, running the initial comparison to support those CMCs seems prudent. Two possible approaches are apparent Use an existing material (such as NIST Ocean Sediment SRM, bentonite from BEV, or some other for which results are already known), for the comparison Develop new material (pilot lab) With natural level of nuclides (to optimize the low level value) With a spiked level of nuclides (easier to measure and to confirm results) Using a pre-existing reference material would simplify preparation and shorten the comparison time. Experience shows that to prepare a reference material for metrological applications takes on the order of 18 to 24 months to acquire the raw material, process it, and analyze its composition, all before samples can be distributed to participants. If an existing material were used, the comparison could be completed within a year. If an existing material is chosen for the comparison, the suite of radionuclides would be pre-determined to be those already contained therein. If a new material is to be prepared, the radionuclides would have to be chosen (probably best done by the willing participants) before preparation (and whether they would be those inherently in the material, added as a spike, or a mixture of the two). 5

6 Sample Handling Once the reference material has been received by the participant, certain general conditions should be followed to assure comparability in measurements (as NIST advises recipients of reference material SRMs). This will be specified in the protocol. Determining Heterogeneity Received material should be tested, using sample sizes of 10 to 50 gram, to determine the heterogeneity of detectable gamma-ray-emitting radionuclides. Uncertainties The contributions to the uncertainty will be, as usual, radionuclide specific. In addition, uncertainty contributions can be expected from any and all of the elements indicated in Table 2, as well as from unique aspects of the specific reference material. Final Recommendations: Soils/sediments represent the largest single type of reference materials currently published in the CMCs. The first reference material comparison, therefore, should support this category (i.e., a comparison of a soil or sediment). A comparison of radionuclide content of reference materials is complicated, at best. If a material were to be developed specifically for such a comparison, the pilot laboratory can expect 2 years or so of effort just to prepare the material for distribution. Therefore, using a pre-existing and pre-characterized material would be preferable. Radionuclides to be evaluated should be decided upon before the beginning of preparations (including production of material) for the comparison. In addition, laboratories should be allowed to declare values for some or all of those radionuclides (i.e., it should not be all or nothing). Minimal sample handling instructions regarding storage, water content ( wet vs. dry weight), and sample size should be provided for the participant, in the protocol. The reporting sheet should be thorough enough to provide all necessary information to assess the methodologies and analysis (including uncertainties) used by the participant laboratory. Uncertainties must be carefully handled not only by each participant, but for the final determination of the reference value. More details on this point should be developed when a comparison is organized. Participants should be gathered from any NMI or MRA signatory (designate) that performs radionuclide analysis of reference materials and who is willing to participant. This should include even those institutions which do not have any related CMCs currently published. If inadequate participation is forthcoming from this pool, the idea of using other capable laboratories (not NMIs) might have to be revisited. Listed in Table 4 are those signatories to the MRA who either claim CMCs for reference materials or who participate in the development of reference material SRMs with NIST. Attached to this document are three proposals for comparisons of radionuclides in reference materials as a starting point in comparisons to support the relevant CMCs. 6

7 Table 4 Laboratory (signatory to MRA) National Commission on Atomic Energy (CNEA) Australian Nuclear Science and Technology Organization Bundesamt für Eich- und Vermessungswesen Institute for Reference Material & Measurements Laboratório Nacional de Metologia das Radiações Ionizantes, Instituto de Radioproteção e Dosimetria Risø National Laboratory (?) Radiation and Nuclear Safety Institut de Radioprotection et de Sûreté Nucléaire Physikalisch-Technische Bundesanstalt International Atomic Energy Agency D.I. Mendeleyev Institute for Metrology, Gosstandart of Russia National Institute of Metrology Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas Institute of Nuclear Energy Research National Physical Laboratory National Institute of Standards and Technology Country Argentina Australia Austria Belgium Brazil Denmark Finland France Germany Monaco Russia PR China Spain Taiwan United Kingdom United States of America 7

8 STATUS REPORT ON CCRI OR RMO COMPARISON 1. CCRI Section: II, measurement of radionuclides 3. Type of comparison: CCRI RMO Key Supplementary Pilot study 2. CCRI Ref No (to be completed by the BIPM): 4. Subject area: Radionuclides in Reference Materials 5. Participating institutes (and countries): International Atomic Energy Agency (Monaco); National Institute of Standards and Technology (USA); open to others 6. Pilot laboratory: National Institute of Standards and Technology (NIST; USA) 7. Measurand / unit (nominal value(s)): Activity per unit mass, mbq/g, nominal value from to 0.6 (nuclide dependent) 8. Description: Assorted radionuclides ( 228 Ac; 241 Am; 212 Bi; 214 Bi; 137 Cs; 210 Pb; 212 Pb; 214 Pb; 238 Pu; 239,240 Pu; 241 Pu; 234 U; 235 U; 238 U; 226 Ra; 90 Sr; 228 Th; 230 Th; 232 Th; 234 Th; 208 Tl) in NIST Rocky Flats II SRM- 4353A (ca. 80 g dried soil) 9. Progress: (Please note date and tick appropriate box to indicate current status) Date Status Pilot Supplementary Key Proposed to CCRI Accepted and registered Protocol submitted to CCRI Protocol agreed Measurements in progress Measurements completed Report agreed Draft A Draft A Report submitted to CCRI Results approved Approved for Equivalence Progression to Key Comparison Abandoned Comments: Publication reference: 10. Measurement start date: 11. Expected measurement completion date: 12. Contact person s name: Lisa Karam Address: NIST 100 Bureau Dr., MS 8460 Gaithersburg, MD USA Telephone: Fax: lisa.karam@nist.gov Web address: Contact Person s signature: 14. Date: 29 March 2005 Completed copy to be forwarded to a) CCRI Executive Secretary, b) Regional coordinator and c) relevant Key Comparison WG Chairman

9 STATUS REPORT ON CCRI OR RMO COMPARISON 1. CCRI Section: II, measurement of radionuclides 3. Type of comparison: CCRI RMO Key Supplementary Pilot study 2. CCRI Ref No (to be completed by the BIPM): 4. Subject area: Radionuclides in Reference Materials 5. Participating institutes (and countries): Institute for Reference Material & Measurements (Belgium); National Physical Laboratory (United Kingdom); Institute of Nuclear Energy Research (Taiwan); National Institute of Metrology (PR China); National Institute of Standards and Technology (USA); open to others 6. Pilot laboratory: National Institute of Standards and Technology (NIST; USA) 7. Measurand / unit (nominal value(s)): Activity per unit mass, mbq/g, nominal value from 0.02 to 2 (nuclide dependent) 8. Description: Assorted radionuclides ( 230 Th; 228 Th; 234 U; 235 U; 238 U; 238 Pu; Pu; 241 Am; 137 Cs) in NIST Shellfish SRM-4358 (ca. 120 g dried shellfish powder) 9. Progress: (Please note date and tick appropriate box to indicate current status) Date Status Pilot Supplementary Key Proposed to CCRI Accepted and registered Protocol submitted to CCRI Protocol agreed Measurements in progress Measurements completed Report agreed Draft A Draft A Report submitted to CCRI Results approved Approved for Equivalence Progression to Key Comparison Abandoned Comments: Publication reference: 10. Measurement start date: 11. Expected measurement completion date: 12. Contact person s name: Lisa Karam Address: NIST 100 Bureau Dr., MS 8460 Gaithersburg, MD USA Telephone: Fax: lisa.karam@nist.gov Web address: Contact Person s signature: 14. Date: 29 March 2005 Completed copy to be forwarded to a) CCRI Executive Secretary, b) Regional coordinator and c) relevant Key Comparison WG Chairman

10 STATUS REPORT ON CCRI OR RMO COMPARISON 1. CCRI Section: II, measurement of radionuclides 3. Type of comparison: CCRI RMO Key Supplementary Pilot study 2. CCRI Ref No (to be completed by the BIPM): 4. Subject area: Radionuclides in Reference Materials 5. Participating institutes (and countries): Australian Nuclear Science and Technology Organization (Australia); Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (Spain); Institute of Nuclear Energy Research (Taiwan); Institut de Radioprotection et de Sûreté Nucléaire (France); National Physical Laboratory (United Kingdom); Physikalisch-Technische Bundesanstalt (Germany); Radiation and Nuclear Safety (Finland); National Institute of Standards and Technology (USA), open to others 6. Pilot laboratory: National Institute of Standards and Technology (NIST; USA) 7. Measurand / unit (nominal value(s)): Activity per unit mass, mbq/g, nominal value from to 1000 (nuclide dependent) 8. Description: Assorted radionuclides ( 228 Ac [ 228 Ra]; 241 Am; 137 Cs; 40 K; 210 Pb; 210 Po; 238 Pu; 239,240 Pu; 239 Pu; 240 Pu; 234 U; 235 U; 238 U; 226 Ra [Bi-214; Pb-214]; 230 Th; 232 Th; 99 Tc) in NIST Seaweed SRM-4359 (ca. 300 g dried seaweed material) 9. Progress: (Please note date and tick appropriate box to indicate current status) Date Status Pilot Supplementary Key Proposed to CCRI Accepted and registered Protocol submitted to CCRI Protocol agreed Measurements in progress Measurements completed Report agreed Draft A Draft A Report submitted to CCRI Results approved Approved for Equivalence Progression to Key Comparison Abandoned Comments: Publication reference: 10. Measurement start date: 11. Expected measurement completion date: 12. Contact person s name: Lisa Karam Address: NIST 100 Bureau Dr., MS 8460 Gaithersburg, MD USA Telephone: Fax: lisa.karam@nist.gov Web address: Contact Person s signature: 14. Date: 29 March 2005 Completed copy to be forwarded to a) CCRI Executive Secretary, b) Regional coordinator and c) relevant Key Comparison WG Chairman